The mechanism by means of which volume-sensitive membrane ion transporters respond to alterations in intracellular content has been studied experimentally and theoretically. A computer program for rapid simulation of many types of experiments that may be performed on the analytical ultracentrifuge has been developed. A new technique for quantification of concentration gradients of multiple proteins in a mixture at sedimentation equilibrium has been developed. Computer programs employing novel algorithms for the global analysis of multiple absorbance gradients of two component solutions at sedimentation equilibrium in the context of models for self- and heteroassociation have been developed.